One of the tougher questions faced by the Guns in the 1950's.
The 25 pounder as a "calibre" was born of necessity. In the 1920's various conferences and boards of experts and ballistics scientists recommended a field gun that had the characteristics of a gun and a howitzer in the 3.5 to 4.5 inch (75 to 100mm) range. The largest possible reboring/resleeving of the 18 pounders of the time led to the 25 pounder at 88mm. So a gun gave birth to a gun/howitzer. Changes in carriage led to the 25 pounder we all know and love... capable of both upper and lower register (high and low angle) and 360 degree traverse.
The 105 is as old a design as the 25 pounder (if not older, the first versions of the 105 had wooden spoked wheels) but was born from a different experience. The Americans conducted what was called the Westerfeld inquiry after WW I and it determined the future of US artillery to the level of detail that its impact is still felt today, 80 years later. The US were prepared to be more adventurous than the UK and didn't try to force the new gun/howitzer to conform to the physical restraints of the French 75 which was the US field gun of the time. They also had experience with the French 105mm howitzer (a Sneider design if I remember correctly) and were much impressed by it ballistics. So saying, the US gave birth to the 105 in essence by getting a howitzer to do gun things, a howitzer/gun, if you like, and the reverse of the UK route.
Both paths led to the same place... guns, as a divisional close support tool, had to evolve into gun/howitzers. There was still a place for pure howitzers and guns at higher levels of support and for general support, but close support went the gun/howitzer route.
Now for the choice between the two. Careful studies of ballistics, time and space considerations, human-machinery interface, ease of maintenance, ammo resupply, etc., showed that the two were pretty closely matched. The 25 pounder fell down a bit with its separate loading ammo, the 105 took a little longer to get into and out of action (the beloved limber trailer won't enter this as by that time most 25 pounders were towed by the truck directly,so the comparisons for time into and out of action were even). The 105 was more forgiving of a rough platform, the 25 pounder needed a flat, relatively level platform. The list of pros and cons for both designs can go on for pages. The point is they were very closely matched.
When it all boiled down, the 105 (for a penalty in weight of ammo for resupply) won out over the 25 pounder as more weight of shell could be dropped on the target by a battery for a slight penalty in other areas. The fact that 105 was to be the NATO standard calibre (if not design) and that we could build the guns in Canada, made the decision all the easier. I wonder how much impact the experiences of the 105mm SP (Priest) regiments in Normandy had on post war deliberations. Their reports from the field were highly complimentary of the 105 and they weren't thrilled when they were converted back to 25 pounder towed.
Now, in a recent discussion with the world renown WWII Gunner, George Blackburn, ("Guns of Normandy", et al) there was no comparison... the 25 pounder was the best design ever made PERIOD. As he pointed out... it is still in service in many parts of the world and was recently fired in anger by Kurdish troops in the Iraq war.
I won't argue with George's opinion. Suffice it to say we are all coloured by our own experiences: I have served on the 105 C1, C2 and C3; its little Italian brother, the L5; and on several variations of the M109. I have restored and carried out gun drill on several 25 pounders and frankly, they are all fine tools for chucking the real weapon of the artillery... the projectile. Personally, I bemoan the loss of the 155 and its earth shaking impact on the bad guys.
Cheers! Mike
